Environmental and regulatory initiatives are requiring increasingly lower levels of sulfur and aromatics in distillate fuels. For example, the proposed sulfur limit for distillate fuels to be marketed as road diesel in the European Union for the year 2005 was 50 wppm. Other regulations require lower levels of total aromatics in hydrocarbons and, more specifically, to lower levels of polycyclic aromatics in distillate fuels and heavier hydrocarbon products. For example, the maximum allowable aromatics levels for U.S. road diesel, CARB reference diesel, and Swedish Class I diesel are 35, 10 and 5 vol. %, respectively. Further, the CARB and Swedish Class I diesel fuels allow no more than 1.4 and 0.02 vol. % polyaromatics, respectively. In California, the use of Ultra Low Sulfur Diesel (ULSD) with a sulfur content of not more than 15 ppmw will become mandatory from 1 Jun. 2006 with a similar constraint to be met everywhere in the U.S. by September 2006. Consequently, much work is presently being done to produce compliant fuels, mainly by hydroprocessing.
Hydrocracking is a well-established method for converting lower value petroleum streams such as FCC cycle oils into more highly refined products with higher hydrogen:carbon ratios and reduced contents of heteroatoms, sulfur and nitrogen. Depending mainly on the desired boiling range of the products, the hydrocracking may be carried out either in a single stage or a two-stage unit. In the two-stage unit, the hydrocracking is typically carried out in a three-reactor, fixed bed unit in which the overall hydrocracking sequence is conducted in the presence of different catalysts each with its own distinct functionality. The feed and added hydrogen are cascaded directly from the initial hydrotreating step to the first hydrocracking step without interstage separation of inorganic heteroatoms (hydrogen sulfide, ammonia). An interstage water wash after the first hydrocracking step is then conventionally carried out to remove ammonia and an amine scrub may also be used to remove hydrogen sulfide from the hydrogen which is then recycled to the unit. One conventional, two-stage hydrocracker uses an initial hydrotreating catalyst in the first reactor (R1), conventionally a CoMo and/or NiMo HDT catalyst. The main portion of boiling range conversion is carried out in the second and third reactors (R2, R3) under more severe reaction conditions using a catalyst which is normally comprised of a Group VIB base metal (Cr, Mo, N, etc.) with one or more Group VIII base metals (Fe, Co Ni, etc.) as promoters on a refractory support such as alumina or silica-alumina, although noble metal catalyst such as palladium or platinum catalysts may also be used; NiW on silica-alumina is a common choice for this catalyst. A post-treat HDT catalyst may be provided at the end of the first stage hydrocracking and also at the end of the second stage hydrocracking reactor zones. The purpose of the post-treat catalyst is mainly to remove reversion mercaptans so as to allow production of low sulfur naphtha for reformer feeds but it has often been found that the hydrocracker diesel frequently contains more than 50 ppm sulfur, a level inconsistent with present or expected regulations. In fact, the achievement of ultra low sulfur levels such as are required by current regulations is difficult even with advances in techniques and catalysts; the difficulty of producing ultra low sulfur diesel fuel has been noted: Hu et al, NPRA Paper No. AM-06-46, (March 2006), “The Era of ULSD—New Challenges and Opportunities for Hydrocracking Processes.”
Hydrotreating catalysts that are particularly suitable for hydrodesulfurization, as well as hydrodenitrogenation, generally contain molybdenum or tungsten, usually the former, on alumina promoted with a metal such as cobalt, nickel, iron, or a combination of these metals. Cobalt promoted molybdenum on alumina catalysts are most widely used when the limiting specifications are hydrodesulfurization, while nickel promoted molybdenum on alumina catalysts are the most widely used for hydrodenitrogenation, partial aromatic saturation, as well as hydrodesulfurization. Recently, a class of nickel-based multimetallic catalysts have been demonstrated to possess very hydrodesulfurization activity when used as hydrocracker pre-treat (first stage) catalysts: the catalysts marketed by Albemarle under the NEBULA™ name have been shown to possess extraordinarily high activities in this service, as reported in “Debottlenecking Hydrocrackers with NEBULA™ Catalyst,” Chitnis et al., NPRA Paper No. AM-05-66.